This invention relates to electromagnetic circuit breakers in which overload sensing is accomplished electromagnetically by a device which permits the circuit breaker to actuate after a time delay period at certain overloads and with substantially no time delay at other overloads. Such devices are illustrated, for example, in U.S. Pat. Nos. 2,360,922 and 3,329,913. These prior art patents disclose electromagnetic sensing devices including a solenoid coil, a time delay tube housing a movable core of magnetizable material movable against the retarding action of a liquid and a pivotal, spring biased, armature. In these devices a time delay is provided upon the occurrence of an overload which, if below a certain value and if it does not persist for a perdetermined time, will not cause the pivoting of the armature and, hence, will not open the circuit breaker contacts, thereby avoiding nuisance tripping. However, in these circuit breakers, if the overload current in a circuit breaker is sufficiently high, the resulting magnetic flux will substantially instantaneously cause the pivotal armature to pivot and, hence, trip the toggle linkage, thereupon opening the circuit breaker contacts substantially instantaneously, i.e., with no intentional time delay.
However, certain electrical loads may safely accept a very high current, usually an inrush current which is a substantially momentary current, i.e., of very short duration, so that at such currents no tripping of the circuit breaker may be desired.
Several attempts have previously been made to modify these known electromagnetic devices to permit the flow through the circuit breaker or relay of ever higher inrush currents, of short durations, without tripping the circuit breakers or the relays, as disclosed in U.S. Pat. Nos. 3,307,130; 3,272,934; 3,517,357 and 3,497,838. It is an object of this invention to provide another arrangement whereby the electromagnetic device will permit the flow therethrough of high inrush currents which are of short durations, without tripping the circuit breakers or relays.